Naruto hasn'mt even shown the full power of Kurama yet, seeing as his first transformation was stated to be quite imperfect. Also, as he showed near the end of the Invasion of Pain Arc, he can combine Sage Mode with Kurama's power for a new mode.
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This mode, I have a feeling, is being saved for the fight against Gedo Mazo which is powered by at least 6, if not 7 Tailed Beasts. Naruto's current state has already put him above basically everyone else in the series and adding Sage Mode would just be crazy.